1. Field of the Invention
Embodiments of the present invention relate to data communication. More specifically, certain embodiments of the invention relate to a method and system for communicating data in xDSL.
2. Background Art
In the field, DSL modems using discrete multitone (DMT) modulations, e.g., Asymmetric Digital Subscriber Line 2 (ADSL2), and Very High Speed Digital Subscriber Line 2 (VDSL2) are typically subject to retrains and/or re-initializations due to sudden variations in noise level. ADSL2 is described in ITU-T Recommendation G.992.3, Asymmetric Digital Subscriber Line Transceivers 2, January 2005, and subsequent Amendments: Amendment 1, September 2005; Amendment 2, March 2006; Amendment 3, December 2006; all of which are incorporated by reference herein in their entireties. VDSL2 is described in ITU-T Recommendation G.993.2, Very High Speed Digital Subscriber Line Transceivers 2, February 2006, and subsequent Corrigenda: Corrigendum 1, December 2006; all of which are incorporated by reference herein in their entireties.
For example, the noise varies in shape and amplitude, lowering the SNR on some tones and causing errors on the link. The modem(s) attempt on line recovery (OLR) procedures (e.g., bit swapping, seamless rate adaptation), but may not recover an error free link for various reasons (e.g., the link is broken in both directions, or the variation would exceed the minimal service requirements such as delay, impulse noise protection (INP), or rate parameters). One modem may essentially give up after a certain length of time, tear down the link, and the modems retrain or reinitialize.
The ADSL2 standard defines a fast initialization sequence to allow the modem to retrain in significantly less time than a normal initialization. This initialization consists in a restart of the modem where the handshake phase is skipped and the duration of the initialization signals are shorter. Handshake procedures are described in ITU-T Recommendation G.994.1, Handshake Procedures For Digital Subscriber Line (DSL) Transceivers, February 2007, and the superseded version of May, 2003 and its Amendments 1-4 (February 2004, June 2004, January 2005, January 2006, respectively); all of which are incorporated by reference herein in their entireties.
During the fast retrain, however, both modems must re-acquire loop timing, retrain equalizers (e.g., time domain and frequency domain equalizers), measure SNRs per tone, and exchange new bit-loading and framing parameters. Fast retrain is not very different than a normal initialization, and thus is not of significantly shorter duration than the normal initialization—except for the deletion of the handshake phase.
What is needed is a system and method to escape steady state (i.e., showtime) and quickly reconfigure or adjust the needed parameters to maintain communication. As used herein, “showtime” is the state of a modem after initialization including training is completed and data (e.g., bearer channel data) is being transferred.